You could also try a more recent browser. Microsoft Edge or Google Chrome are probably the easiest; they both have automatic updates that let you download and apply the latest patches. If you are using Chrome and using the Acrobat Reader, we suggest that you try the latest version, which is 21 October 2017. Otherwise, try to use these steps: Please try to combine the PDF files with the URL on a different browser like Chrome, or Firefox, or Safari, and see if this 1- Update the Acrobat (Reader DC) and Adobe Reader If you are using Adobe Reader DC, restart your computer and try to combine the PDF files, and you might have to press the Escape key on the keyboard to quit Acrobat for Windows. 2- Update Adobe Reader in the OS X To save your hard drive space, please download Adobe Portable Document Format (PDF) Converter from the web (download Adobe Reader DC from here): Install Adobe Portable Document Format Converter and Adobe Reader DC from Applications > Adobe > Portable Document Format Converter It is recommended to use the latest version of Adobe Reader (the 11.5.2 patch for desktop) because Adobe's PDF Reader doesn't support creating 'New Document' which is an important feature that the 'Combination' option. Download Acrobat Reader DC and Acrobat Reader for Windows: The PDF format does not support 'New Document'. For the best viewing performance, the 'Combination' PDF format requires that the PDF documents are saved to a folder within the user's home directory which are known as 'Combined files'. If you are using Acrobat Reader for Windows 10, you should download and install the latest version of Adobe Reader. See, Please note that we DO NOT recommend installing these programs using Software Update of macOS. When the latest patch for Adobe Reader or Acrobat Reader DC is used, they will overwrite the 'Combination' option and create a new 'Documents' folder that won't allow you to create new files/folders. Please refer to the Adobe website for details. If you are using one of the previous versions of Acrobat, you can try the latest Acrobat Reader. Please note that in the version 11.5.2 the 'Combination' option didn't work.